1. Field of the Invention
This invention relates to a fabrication method for a semiconductor device, more particularly, to creating an activatable conducting link in an integrated circuit (IC), to replace bad circuitry with spare or replacement circuitry previously fabricated in the IC.
As the degree integration of ICs grow, the necessity for redundancy within the chip increases in order to enhance device yield. The benefits and necessity for component redundancy have been recognized and recently become economically justifiable by the manufacturers. Redundancy is implemented by providing an IC, for example, a memory chip (or die) with spare circuitry such as spare rows or spare columns of memory cells and increases chip yield.
The replacement of bad circuitry is accomplished by melting a fuse connecting the bad circuitry and creating conducting links associated with the spare circuits. The present invention is involved in the method of creating the activatable conducting links.
2. Description of the Prior Art
Usually, a non-conductive link is formed between specified points in the circuit and when it is necessary to link the points, spot heating changes the non-conductive link to a conductive link. The method of creating links in this manner is set forth, for example, in 1981 IEEE International Solid State Circuit Conference Report, page 14, by O. Minato et al.
FIG. 1 is a cross sectional view of a prior art activatable link. An insulating layer 2, such as silicon dioxide (SiO.sub.2), is fabricated on silicon substrate 1. On the insulating layer 2, polycrystalline silicon (polysilicon) conducting paths 3, which are highly doped with an N-type dopant, such as phosphorous, are formed separated from each other by an non-doped polysilicon insulating gap 4 about 1 .mu.m wide and having a resistivity of more than 10.sup.9 .OMEGA.cm. The above-described structure is covered with a phospho-silicon glass (PSG) sealing layer 5 to protect and passivate the surface. When an electrical link is required between the separated conducting paths 3, the insulating gap 4 is "activated" by laser irradiation. The irradiated portion of the insulating gap 4 is heated up to around 1400.degree. C., and the dopant contained in the separated conducting paths 3 is diffused into the insulating gap 4, reducing its resistivity down to approximately 10.sup.3 .OMEGA.cm. FIG. 2 illustrates a cross sectional view of a conducting link formed by such a process.
There are some undesirable effects resulting from the heating in the process described above. The heat generated by the laser irradiation is transferred to neighboring areas, and the characteristics of the semiconductor device near the spot are affected due to the re-arrangement of the dopant. The surface of the conducting paths 3 where it is exposed from under the sealing layer 5 is oxidized, and, as a consequence, the reliability of the paths 3 is reduced. Furthermore, the PSG sealing layer 5 trends to be porous and the surface becomes rough, collecting dust and moisture, causing insulating problems. In order to activate the conducting link, the temperature of the laser irradiated portion must be raised up to about 1400.degree. C., which requires relatively high laser power to obtain such a high temperature. As long as the conducting link is created by the diffusion of a dopant into insulating gap, the high temperature heating is unavoidable.